Depending on the stereo-structure, polypropylene is classified into isotactic polypropylene (hereinafter, referred to as “iPP”), syndiotactic polypropylene (hereinafter, referred to as “sPP”), and atactic polypropylene (hereinafter, referred to as “aPP”).
Relative to the iPP and sPP which have good mechanical properties and thermal characteristics and thus have been actively exploited, the aPP is still undeveloped in terms of commercial use due to its low isotacticity and consequent limitation in enhancement of properties.
The aPP can be isolated as a by-product during the recovery of an aliphatic solvent in the slurry process for preparation of iPP, or prepared in the presence of a heterogeneous catalyst containing a modified titanium (III) chloride and an organoaluminum compound, such as diethyl aluminum chloride, as a co-catalyst or an activator. However, the iPP process with improved isotacticity cannot yield an amorphous aPP as a by-product but needs the addition of a co-monomer even when a PP with low crystallinity is produced according to the use purpose.
There has recently been a suggestion to a method for preparing an aPP with narrow molecular weight distribution using a variety of metallocene catalyst systems. The metallocene catalyst systems useful in this method are, for example, (a) an achiral, unbridged metallocene or stereo-rigid C2v symmetric metallocene catalyst, (b) meso-isomeric ansa metallocene catalyst, (c) an ansa-C2 symmetric metallocene catalyst with a bridge at 2,2′-position of the indenyl ring, and (d) a catalyst of monoCp compound.
The properties of the aPP greatly change according to the molecular weight of the aPP. For example, the aPP with a weight average molecular weight below 15,000 is sticky at the room temperature and thus has a limitation in its use, consequently with low applicability as a polymer material.
Further, the conventional metallocene catalyst systems have a low catalytic activity for polymerization to prepare a polypropylene and require a relatively low polymerization temperature (for example, 20° C. or below) enough to acquire an aPP with high molecular weight.